(1) Field of the Invention
The present invention relates to sensors, and particularly to a sensor which senses contact between a probe and an object.
(2) Description of the Related Art
FIG. 9 is an external perspective view showing an example of a commonly-used conventional turret lathe. As shown in the figure, a turret lathe 1 includes a spindle 2 and a turret 3.
The spindle 2 is supported by a headstock 5 provided on a head 4, and holds and rotates a workpiece (not shown) through a spindle chuck 6.
The turret 3 includes a turret tool post 7, an indexing mechanism 8, and a turret slide 9.
The turret tool post 7 has tool holding faces 10 around a rotating shaft, and is rotatably supported by the indexing mechanism 8. On each the tool holding faces 10, a tool 11 such as a tool bit or drill is inserted via a tool station 11a. 
It should be noted that the straight line connecting the rotational axis of the spindle 2 and the rotational axis of the turret tool post 7 is parallel with the X-axis (in the horizontal plane) shown in the figure.
The indexing mechanism 8 is provided on the turret slide 9, and, through a motor, rotates the turret tool post 7. As a result, the indexing mechanism 8 positions a specific tool holding face 10 on the workpiece side.
FIG. 10 is an elevation view showing a positional relationship between the turret tool post 7 and the spindle 2 of the turret lathe 1. FIG. 11 is an elevation view showing the movable range and maximum turning radius R of the turret tool post of the turret lathe.
As shown in FIG. 10, a tool 11 selected for the machining of a workpiece 12 is aligned to a cutting position by being rotated at the maximum turning radius R of the turret tool post 7. Here, a cutting position is a position at the workpiece 12 side, which is on the straight line which passes the rotational axis of the spindle 2 and is parallel to the X-axis.
The indexing mechanism 8 can, through a motor, move the turret tool post 7 in a Z-axis direction which is a direction that is parallel to the rotational axis of the spindle 2.
The turret slide 9 is provided on the bed 4 to be movable along the X-axis direction. The turret slide 9 moves along the X-axis direction together with the turret tool post 7 and the indexing mechanism 8 through a motor. As a movable range, the turret slide 9 moves through a range in which the tool 11 extends slightly beyond the rotational axis of the spindle 2.
Through the indexing mechanism 8 and the turret slide 9, the turret tool post 7 can move in the X-axis direction and the Z-axis direction. Therefore, the indexed tool can perform machining on the workpiece 12 held in the rotating spindle 2, in the radial direction and the axial direction.
The above described structure is normally placed in a housing (not shown), and workpieces are machined therein.
Here, some machine tools such as turret lathes have a function for measuring the dimensions of a workpiece in the machine tool, without having to remove the workpiece from the spindle. This function enables the dimension measurement to be performed without having to take a workpiece out of the machine tool, and thus the work efficiency of the machine tool improves. Further, by measuring displacement of workpieces and/or displacement of the machine tool caused by changes in the in-machine environment such as a variation in the temperature, and by calibrating a control amount for the machine tool according to the measured displacement, it is possible to ensure highly accurate product dimensions.
For example, a turret lathe 1 is available which has a function for measuring the diameter, and so on of the workpiece 12, using a sensor mounted on a tool holding face 10 of the turret tool post 7, through a tool station 11a or through similar fitting equipment.
Furthermore, as disclosed in Japanese Unexamined Utility Model Application Publication No. 07-3902, a turret lathe has been proposed which has a mechanism for directly measuring the diameter of a workpiece by measuring the position of both ends of the workpiece.
FIGS. 12A and 12B are schematic diagrams each showing an example of the conventional turret tool post 7 having a mechanism for directly measuring the diameter of the workpiece 12. FIG. 12A is an anterior view and FIG. 12B is a top view.
The turret lathe 1 shown in FIGS. 12A and 12B includes, at the front of the turret tool post 7, an arm 15 which is rotatably supported by a non-rotating member at the center of the turret tool post 7. Further, a sensor 20 is mounted at the tip of the arm 15.
The length of the arm 15 is sufficient for the sensor 2 to measure the position of one end (P2) of the diameter of the workpiece 12 out of both ends (P1 and P2), which is on a side opposite to the turret tool post 7 with the rotational axis of the spindle 2 located therebetween, when the turret tool post 7 approaches the spindle 2.
By adopting such a structure, the turret lathe 1 is capable of measuring the positions of both ends P1 and P2 of the diameter of the workpiece 12 as shown in FIG. 12B. The diameter of the workpiece 12 can be derived from the difference between P1 and P2 obtained in such a manner.
The structure of the conventional sensor 20 used in the in-machine measurement of such turret lathes shall be described with reference to the drawings.
FIG. 13 is a cross-section view showing an example of the structure of a conventional sensor used in in-machine measurement.
The sensor 20 is a sensor which senses contact with an object through the tilting of a rod-shaped probe, and includes a housing 22, a probe 24, a connectors 26a, 26b, and 26c, terminals 28, 29, and 30, and a spring 25.
The housing 22 is a hollow columnar housing which internally houses the respective parts. The housing 22 is mounted directly onto fitting equipment such as the tool station 11a. 
The probe 24 is a rod-shaped rigid body that is positioned so as to project from the inside to the outside of the housing 22, and is provided so as to allow tilting with respect to the housing 22.
The spring 25 has a biasing force in a direction which distances the probe 24 from one surface of the housing 22. Furthermore, although such biasing force pushes the connectors 26a, 26b, and 26c into contact with the terminals 28, 29, and 30, respectively, the biasing force is such that the contact between at least one of the connectors 26a, 26b, 26c, and terminals 28, 29, 30 is released by even the slightest contact between the tip of the probe 24 and the workpiece 12.
The connectors 26a, 26b, and 26c are three columnar metal members that can maintain conduction through contact with the terminals 28, 29, and 30, respectively, and which are fixed to the middle portion of the probe 24. The connector 26a (26b, 26c) is fixed perpendicularly with respect to the axis of the probe 24. The connectors 26a, 26b, and 26c are arranged evenly around the same circle, and the angle between neighboring connectors 26a and 26b (26b and 26c, 26c and 26a) is 120 degrees. In other words, the connectors 26a, 26b, and 26c are fixed to the probe 24 so as to form an even radial pattern with respect to the axis of the probe 24.
The terminal 28 (29, 30) is a member that can maintain conduction through contact with the connector 26a (26b, 26c), and is configured of two columnar electrodes 28a and 28b (29a and 29b, 30a and 30b) which are arranged in the shape of the letter V. The two columnar electrodes 28a and 28b (29a and 29b, 30a and 30b) are insulated from each other and from the housing 22, and are mounted on an inner end face of the housing 22, with the open end facing inward. The three terminals 28, 29, 30 are placed in positions corresponding to those of the connectors 26a, 26b, 26c, respectively.
Next, the sensing method of the sensor 20 shall be described.
The sensor 20 has a wiring connection as shown in FIG. 14. More specifically, the electrode 28a (28b) configuring the terminal 28 is wire-connected to the electrode 29a (30a) of the neighboring terminal 29 (30) by a conducting wire 31a (31b). In addition, the connector 26a (26b, 26c) is pushed into contact with the terminal 28 (29, 30). Therefore, as shown in FIG. 15, the two electrodes 28a and 28b are electrically connected through the connector 26a. The two electrodes 29a and 29b (30a and 30b) configuring the other terminal 29 (30) are also electrically connected through the connector 26b (26c) in the same manner. Thus, there is conduction between two outgoing wires 32a and 32b. 
Next, when the probe 24 comes into contact with the workpiece 12, the probe 24 tilts with respect to the housing 22. At least one of the connectors, for example, 26a (26b, 26a) fixed to the probe 24 rises from the corresponding terminal 28 (29, 30), with the corresponding remaining terminals 26b and 26c (26c and 26a, and 26a and 26b) as axes. With this, the two columnar electrodes 28a and 28b (29a and 29b, 30a and 30b) become insulated from each other and thus the two outgoing wires 32a and 32b become insulated from each other.
Therefore, by observing whether or not the two outgoing wires 32a and 32b are insulated from each other, it is possible to sense contact between the probe 24 and the workpiece 12. With this, the turret lathe can obtain information indicating the position in the X-axis direction at which the sensor comes into contact with the workpiece, and the dimensions of the workpiece can be measured based on the obtained position information.
However, when a sensor is mounted on a turret tool post, a problem has been discovered in which the operational life of the sensor is significantly shortened due to the vibration generated during the rotation of the turret tool post, the cutting of workpieces by the cutting tools mounted on other positions in the turret tool post, and so on.